The final round in Afghanistan's presidential election has begun as voters headed for the polls to pick a successor to President Hamid Karzai who has been in power since the US-led military invasion ousted the Taliban in 2001.

Afghan voters started casting their ballots across the war-stricken country at 7 a.m. (0230 GMT) on Saturday, Reuters reported.

The vote pits former anti-Taliban fighter Abdullah Abdullah against ex-World Bank economist Ashraf Ghani after neither secured the 50 percent majority needed to win outright in the first round on April 5.

In the first round of voting in April, Abdullah took the lead with 44.5 percent of the vote, with Ghani trailing behind at 31.5 percent. The elections drew a surprisingly large turnout - more than 7 million people participated out of an estimated 12 million eligible voters.

Several Afghan officials who were not given permission to speak to the press told Al Jazeera they fear ethnic and sectarian divisions in the government - despite both candidates trying to sell a vision of a broad-based coalition.

"Our biggest concern is that neither of these candidates has the power to bring people together like Karzai," one government source said. "Ghani’s people are looking to work more for the Pashtuns and if we see Abdullah, well, he will bring more Tajiks to the palace. This is a big concern for officials at the moment."

Mahmoud Saikal, an adviser to Abdullah, said stability, including the political transition and peace process, security and economic development are Abdullah’s three main focuses, along with corruption, which he referred to as "the mother of them all".

In an interview to the AFP last year, Ghani said he envisions an "economically interdependent" Afghanistan that "serves as a crossroad of a vibrant Asia continental economy". However, he stressed, the next five years are going to be difficult.

"First we need to tackle our widespread corruption, managing this is going to require an immense amount of effort, will and concentration," Ghani said.